1. Field of the Invention
The present invention relates to a link adaptation method and apparatus in wireless communication system; and, more particularly, to a method for measuring received signal strength and a method for controlling a transmission parameter according to the measured received signal strength.
This work was supported by the IT R&D program for MIC/IITA [2006-S-014-02, “Development of IEEE 802.11n Modem & RF Chip-sets with Data Rate 200 Mbps”].
2. Description of Related Art
In a wireless communication system, a channel state is changed by causes such as interference, noise, propagation route, and attenuation to thereby deteriorate the performance of the wireless communication system and make normal communication impossible. Accordingly, a link adaptation method for controlling a transmission parameter according to a wireless channel state and providing communication optimized to the channel state has been studied.
There are three conventional link adaptation methods. First, there is an auto rate fallback (ARF) method which is most generally used. The ARF method is a method for sensing packet reception failure through cyclic redundancy checking (CRC) and lowering a transmission speed by one step when a packet reception failure value is higher than a predetermined value. The ARF method can be easily realized but does not follow the change of the channel and is not helpful in entire system throughput.
Second, there is a Signal-to-Noise Ratio (SNR) method. The SNR method is applied when finely divided Media Access Control Protocol Data Unit (MAC MPDU) is transmitted by measuring the SNR upon receiving the packet. The SNR method has a fast response speed for channel change but it is complicated to realize hardware to exactly measure the SNR.
Third, there is a method using received signal strength (RSS). Since the received signal strength has linear relationship with the SNR on average, the channel state is estimated by measuring strength of the received signal. Accordingly, a transmitting end can determine and transmit a transmission parameter. Subsequently, a receiving end receives the transmission parameter, measures received signal strength and transmits the received signal strength after link adaptation. These procedures are repeated in this method.
The method using the received signal strength has the fastest response speed through adaptation to the channel change on the basis of a packet unit. However, since the receiving end should measure strength before a signal received through an antenna is amplified, there is a problem that hardware complexity increases by connecting an individual analog-to-digital (ADC) converter prior to an amplifying unit. Also, when a signal exceeding a conversion threshold range or too small signal is inputted, it is difficult to exactly measure the received signal strength.